The concept of separating pulps in two different fractions and treating the two fractions differently is not uncommon. For example, such separation of mechanical pulps into a long fiber fraction and short fiber fraction and treating the two pulps differently or treating only the long fiber fraction is taught for example, in Canadian patents 1,177,607 and 1,177,608 issued to Mackie et al. on Nov. 13, 1984. These processes have not attained significant commercial success.
Recovery of fiber from used corrugated boxes generally known as old corrugated containers (OCC), and its reuse to produce duplex linerboard is well known. In these processes, generally the OCC is first repulped and then separated into a base sheet fraction and a top sheet fraction which are then separately refined to develop the characteristics desired for the particular application to which the pulp is to be applied. If the pulp is to be used as a top or exposed sheet, it will be subjected to significantly more beating or refining than the coarse or base sheet.
Sometimes, the separation of the pulp into two fractions is done in a fractionator to produce long fiber fraction and short fiber fraction which are again refined separately to produce the required characteristics for the particular pulp, i.e. the short fiber fraction requires less work than the long fiber fraction although the short fiber fraction is not normally used for the top sheet. The objective is to have a base sheet with a freeness of somewhere between 500 and 600 and top sheet with a freeness of somewhere between 250 and 400. The resultant sheets generally have about the same physical characteristics. However, the fines fraction seems to be a little brighter and thus, improves the brightness slightly of the exposed surface.